Field of the Invention
This invention relates to transfer rollers for electrophotographic apparatus and more particularly to a transfer roller for electrophotographic apparatus which can repeat development and transfer with respect to an electrostatic latent image which has been produced on a latent image charge holding body and which can obtain a plurality of copies.
Heretofore, it has been the common practice to use an electrophotographic apparatus that can obtain one copy from an electrostatic latent image which has been produced on a latent image electric charge holding body such as a photosensor. Electrophotographic apparatus of this type, hereinafter called single-copy electrophotographic apparatus, generally makes use of a transfer device based on a corona discharge transfer system or bias roller transfer system.
Meanwhile, an electrophotographic apparatus has heretofore been proposed which can repeat development and transfer with respect to an electrostatic latent image which has been formed on a latent image electric charge holding body to obtain a plurality of copies. Electrophotographic apparatus of this type hereinafter called multiple-copy electrophotographic apparatus, preferably uses a transfer device based on the bias roller transfer system rather than that based on the corona discharge transfer system. The use of the transfer device based on the bias roller transfer system ensures a slight disturbance of the electrostatic latent image when the record sheet makes contact with and is separated from the latent image electric charge holding body. Nevertheless, it is difficult to bring the record sheet into close contact with the latent image electric charge holding body, thereby inducing a transfer failure. In order to eliminate such drawback, the transfer bias voltage must be made high. When the transfer bias voltage is made high in order to improve the transfer efficiency, the single-copy electrophotographic apparatus operates without problem, but the multiple-copy electrophotographic apparatus has the drawback that, after a single occurrence of the following phenomenon, all of succeeding copies deteriorate. That is, if the transfer bias voltage is made high, the electric charge injected from the transfer roller through the record sheet into the latent image electric charge holding body is increased and accumulated on the surface of the latent image electric charge holding body. This injection electric charge is repeatedly produced in succession at every transfer during the step of obtaining a plurality of copies and does not exert any adverse influence upon the quality of the picture image during the first step of obtaining 2 to 3 copies. But, in the case of obtaining a large number of copies such as 5 to 10, particularly 20 to several tens of copies, the accumulation of the injection electric charge is gradually increased.
At the time of development, the toner is adhered to the injection electric charge and then, during the transfer step, the toner is transferred to the record sheet. As a result, contamination of the record sheet due to the toner gradually increases. This phenomenon will hereinafter be called as a multiple-copy fog phenomenon. Thus, it is impossible to obtain a large number of copies having good quality. The higher the transfer bias voltage the more conspicuous this phenomenon. If the transfer bias voltage is further increased, a discharge phenomenon occurs when the record sheet is peeled off the latent image electric charge holding body. As a result, a troublesome electric charge is generated due to the discharge in the background within the region of the picture image of the latent image electric charge holding body and this electric charge is developed, thereby obtaining unusable, dirty copies only. As the number of transfers is increased, the electric charge due to the discharge is accumulated so that it is impossible to obtain copies which are suitable for practical use. In order to remove the undesired electric charge due to the discharge, all of the surface of the latent image electric charge holding body may be exposed to light during the step of obtaining a plurality of copies. But, in this case, it is impossible to obtain any subsequent copies unless the electrostatic latent image is produced again.
If the transfer bias voltage is further increased, the discharge becomes considerably large when the record sheet is peeled off the latent image electric charge holding body. As a result, the toner image transferred to the record sheet is disturbed, the toner image on the latent image electric charge holding body is disturbed prior to its transfer to the record sheet or the electrostatic latent image is disturbed, thereby deteriorating the latent image electric charge holding body per se in itself.
In the single-copy electrophotographic photographic apparatus, the upper limit of the transfer bias voltage that is applicable to the transfer roller is defined by a voltage value at which the toner image is disturbed due to the above mentioned discharge phenomenon. In the multiple-copy electrophotographic apparatus, the upper limit of the transfer bias voltage that is applicable to the transfer roller is defined by a voltage value at which the above mentioned multiple-copy fog phenomenon occurs. This voltage value is far lower than the voltage value at which the above mentioned discharge phenomenon occurs. As a result, the conventional transfer roller has the drawback that its transfer efficiency is bad and that a plurality of copies cannot be obtained at a high speed.